1. Field of the Invention
The present invention is broadly concerned with an improved, low cost method for removal of arsenic and other heavy metals from phosphoric acid in order to obtain a greatly purified product. More particularly, it is concerned that such a method which involves, inter alia, a liquid extraction of heavy metal sulfides, preferably through the use of carbon disulfide as an extractant.
2. Description of the Prior Art.
Phosphoric acid is produced on a commercial scale by two methods, namely the furnace process and the wet process. Furnace acid is generally purer than wet process acid (the latter sometimes being referred to as "green acid"), but is significantly more costly to produce. Indeed, furnace grade acid generally is designed for a more specialized market requiring a high degree of purity.
Both types of phosphoric acid generally contain as contaminants heavy metals, and particularly arsenic, lead, copper, and bismuth. Arsenic makes up the very large majority of heavy metal species in most phosphoric acids. These contaminants typically exist in the phosphate ore used as a starting material, and are carried through both commercial processes, thus becoming contaminants in the final acids. The typical concentration of heavy metals in furnace grade acid is from about 50-200 ppm, depending upon the quality of ore employed.
Heavy metal contaminants, and particularly arsenic are very undesirable in phosphoric acid, particularly acids destined for the food grade market. As a consequence, producers of high quality acid take steps to remove the heavy metal contaminants during processing. Almost without exception, such heavy metals are removed by a sulfide treatment of the acid, involving reacting the acid with hydrogen sulfide or a sulfide salt such as sodium sulfide. The resulting heavy metal sulfides tend to form a very fine particulate suspension in the liquid acid, which can be extremely difficult to remove. Accordingly, processors make use of filter aids such as diatomacous earth, perlite, or bentonite clay, which increase filtration retention and/or throughput. Consequently, the filter aids are removed with the heavy metal sulfides as a filter cake.
While the above-described process is capable of lowering the heavy metal content of phosphoric acid to less than 10 ppm (with an arsenic content of less than 3 ppm), the resultant filter cake is considered a hazardous waste, and therefore disposal of the filter cake can be very difficult and expensive. At the same time, the filter cake tends to have considerably greater volume than what would be predicted by the stochiometry of the overall process as it contains phosphoric acid, water, and filter aids in addition to the heavy metal sulfides.
Accordingly, there is a real and heretofore unsatisfied need in the art for an improved process for removing heavy metal contaminants (and especially arsenic) from wet or furnace grade acid which eliminates or at least minimizes the disposal problems inherent in the use of conventional filtering aids.